The present invention relates to an impedance-to-voltage converter (hereinafter referred to as xe2x80x9ca Z/V convertersxe2x80x9d) for outputting a voltage corresponding to an impedance value of a target to be detected using an operational amplifier. More particularly, the present invention relates to a Z/V converter using an operational amplifier, which can remove the influence of stray capacitances on a signal line, and thereby can produce a voltage corresponding to an impedance of a target to be detected all the more accurately.
FIG. 1 schematically illustrates a prior art electrostatic capacitance-to-voltage converter described in Japanese Patent Public Disclosure (Laid-open) No. 61-14578. This electrostatic capacitance-to-voltage converter has been proposed to solve the following problems caused by stray capacitances on a cable connecting an unknown electrostatic capacitance to an input terminal of an operational amplifier. That is, the stray capacitances on the cable are superimposed on the electrostatic capacitance to be detected and the values of the stray capacitances vary due to movements, bending and so on of the cable, so that the impedance value of the electrostatic capacitance may not be converted into a correctly-associated voltage.
As shown in the prior art, when the value of the unknown electrostatic capacitance Cx is smaller, the influence of stray capacitances on the lines becomes prominent, thereby causing a problem that the converter is prevented from correctly converting the capacitance Cx into a voltage. In addition, in a case where one electrode of the capacitance Cx is biased to a certain voltage, no AC signal can be applied to the capacitor Cx, thereby causing another problem that the capacitance Cx cannot be converted into a voltage at all.
The present invention has been proposed to solve such inherent problems in a prior art as shown in FIG. 1. Therefore, it is an object of the present invention to provide an impedance-to-voltage converter (Z/V converter) which is capable of highly accurately converting an impedance value Z of a target or a component to be detected into a voltage V without any influence of stray capacitances occurring between a signal line and a shielding means, even if the impedance value Z is relatively small.
In addition, it is an object of the present invention to provide a Z/V converter which is capable of highly accurately converting the impedance value Z of a target or a component to be detected into a voltage V without influence by stray capacitances between a signal line and a shielding means, even if one electrode of the target is kept at a certain voltage.
In order to achieve objects mentioned above, an impedance/voltage (Z/V) converter for converting an impedance of a target to a voltage comprising an operational amplifier having a feedback impedance circuit connected between an output terminal and an inverting input terminal thereof, a signal line having one end connected to the inverting input terminal of the operational amplifier and the other end connected to one electrode of a target impedance, alternating current (AC) signal generator connected to a non-inverting input terminal of the operational amplifier, and at least one shields comprising at least one shielding layer which surrounds at least a portion of the signal line and is connected to the non-inverting input terminal of the operational amplifier and the AC signal generator, wherein the inverting and non-inverting terminals are imaginal-short, whereby the operational amplifier outputs from its output terminal a voltage corresponding to the impedance value of the target.
The shielding layer preferably comprises a mesh structure or tube structure. In addition thereto, it is preferable that the shield further includes a second shielding layer surrounding the outer surface of the first shielding layer, which comprises a mesh structure or tube structure and is connected to the non-inverting input terminal of the operational amplifier and the AC signal generator or to a reference voltage.
Preferably, the impedance of the target and the feedback impedance circuit both have the same character comprising a resistive, capacitive or inductive one or any combination thereof. In such a case, S/N ratio of the apparatus may be improved. Other combinations can be acceptable and when the impedance of the target is an electrostatic capacitance, and the feedback impedance circuit is a resistance, it is easy to integrate the operational amplifier and the feedback impedance circuit in a chip.
When a generator of a DC voltage corresponding to the impedance value of the target is provided, further processing is made easier. It is also possible to modify the Z/V converter such that the feedback impedance circuit is a second target impedance which is unknown, in which case an output voltage of the operational amplifier corresponds to the ratio of the first and second targets impedances.